The present invention relates to an apparatus for testing workpieces, in particular automotive vehicle tires, by means of X-rays, as defined in the introductory portion of patent claim 1.
European patent application 0 315 099 discloses an apparatus for multi-lateral X-ray testing of automotive vehicle tires, wherein an omni-directional X-ray tube disposed in proximity to the interior of a tire is used to transilluminate the side walls and tread of the tire. Outside of the tire to be tested there is provided a U-shaped assembly of three diode arrays receiving the X-rays from the tire. Each diode array comprises a linear line of light-sensitive diodes which are spaced from each other for 0.45 mm or even 0.225 mm. The receiving surfaces of the diodes are covered by a fluorescent layer. The light generated by the X-rays in the fluorescent layer is detected by the light-sensitive diode providing an electrical signal in response thereto. The diodes of the diode arrays are scanned by a scanner, and the output signals of the diode are transmitted to a store. An image reproducing device comprising a plurality of individual picture elements serves to generate image lines for each scanning sequence. Accordingly a predetermined surface portion of the tire to be tested is reproduced on the image screen of the diode arrays. The apparatus known from the European application enables a substantially vertical transillumination of all areas to be tested and thus a distortion-free reproduction on the reproduction apparatus. The diode arrays are adjustably mounted as to their spacing from the tire so that a uniform reproduction scale may be obtained for different tire sizes, which substantially facilitates testing of a tire by an operator. Of further advantage is that a screen reproduction is possible even with tire enforcements of plastic material (plastic cord) despite its poor contrast characteristics.
The light sensitive diodes including their electronic circuitry are received in a hermetically sealed housing so as to protect the sensitive elements from environmental disturbances, in particular dust. While the housing requires a through-slot for the X-rays, this through-slot is closed by an X-ray transmissive material so as to avoid absorption of the X-rays. Furthermore the housing area on both sides of the through-slot are provided with a lead-shield which is intended to prevent the X-rays penetrating through the housing walls from affecting or even destroying the electronic circuitry of the diodes.
In the prior art apparatus the dimension of the housing along the axis of radiation is relatively substantial, with the diodes being positioned adjacent the housing wall opposite the through-slot. The width of the through-slot cannot be made extremely small due to the fact that the incident radiation is to extend in the plane of the diode line. Manufacturing tolerances of the housing and of the other testing apparatus, accordingly require a minimum width of the through-slot. As a result thereof there is a certain risk that the areas adjacent to the diodes are also hit by X-rays, and sensitive parts an components may be detrimentally affected, and their life time will be substantially reduced. Furthermore the X-rays penetrating the housing across a substantial width may cause scattered radiation in the housing resulting also in a reduction of the life time of the electronic components.
By using small spacings of the diodes of a diode line a high resolution may be obtained so that e.g. tire re-enforcements of plastic fibres may be tested. In the art diode lines having a spacing of 0.2225 mm are known. The extent of the receiving surface of such diodes transverse to the diode line, however, is substantially greater for technical reasons and is presently about 0.6 mm. If a textile is to be tested wherein the fibres extend substantially parallel to the diode line and are spaced from each other for a distance that is smaller than the extent of the diode transverse to the diode line, there is a certain risk that a pair of fibres is simultaneously reproduced on the receiving surface An electric average value will result therefrom, and a proper resolution is not ensured any longer.
X-ray testing of e.g. automotive vehicle tires has been based on the fact that the tire basic material and the tire re-enforcements have different absorbing characteristics with respect to X-rays. So it is possible to obtain a relatively rich contrast between the tire basic material and e.g. steel re-enforcements. Furthermore it has become known to use textile re-enforcements (plastic cord) in tires, in particular in the tire side walls. Such re-enforcements have a reduced density as compared to rubber and provide only a relatively poor contrast. This is why tires including both steel and textile re-enforcements may not be tested in a single testing step. In order to obtain a sufficiently high resolution with respect to plastic cord, the X-ray tube must be operated at a relatively low voltage. This voltage would not be sufficient to make visible the structure of steel reenforcements. So it would be necessary to use different voltages depending on which type of tire re-enforcements are to be tested.